SiO2-coated lead halide perovskites core–shell and their applications: a mini-review

Research on lead halide perovskites has demonstrated that they are one of the potential materials for optoelectronic and bio-related applications owing to their promising optical and electronic properties. However, their poor chemical stability in ambient environments is a critical factor that affects their practical applications. Silica is known for its excellent environmental/chemical stability and good optical properties. Therefore, SiO2-coated lead halide perovskites have been studied by introducing the protective layer containing SiO2 to prevent the rapid destruction of their surface chemistry and environmental degradation. It is found that lead halide perovskite core–shell can significantly improve the stability and preserve their high photoluminescence quantum yield. In addition, controlling the shell thickness is also important to produce effective and suitable inorganic halide perovskites core–shell for practical applications. This mini-review discusses the stability, synthesis method and applications of SiO2-coated lead halide perovskite core–shell. Furthermore, the effect of the SiO2 shell thickness on lead halide perovskite core–shell-based applications is also reviewed.


Introduction
In recent years, lead halide perovskite materials with the structure ABX 3 (where A is caesium, B is a lead cation and X is a halide anion) exhibit numerous advantages such as high brightness, and high photoluminescence quantum yield (PLQY) with narrow size distribution, which can provide great potential in optoelectronic devices [1].These lead halide perovskite-based 3. SiO 2 -coated lead halide perovskites 3.1.Tetramethyl orthosilicate/tetraethyl orthosilicate-based lead halide perovskites core-shell Zhong et al. [41], Park et al. [43] and Chen et al. group [44] successfully reported the one-spot synthesis of CsPbBr 3 @SiO 2 core-shell nanoparticles by adding caesium bromide, PbBr 2 , oleic amine (OAm), oleic acid (OA), dimethylformamide (DMF) and ammonia solution into toluene containing tetramethyl orthosilicate (TMOS) (figure 1a).This synthesis method was carried out without the requirement of an inert atmosphere and heat treatment.These characteristics can ensure better reproducibility and reduce costs in large-scale production.Thanks to the silica layer acting as a protective shell, the stability of CsPbBr 3 @SiO 2 in humid air (25°C and humidity of 75%) and water was dramatically improved.The X-ray diffraction (XRD) signal of the SiO 2 -unprotected CsPbBr 3 disappeared due to the decomposition of the core.In contrast to the XRD signal of CsPbBr 3 , the intensity of XRD peaks from CsPbBr 3 @SiO 2 core-shell remains the same after 3 days.In the case of stability in water under harsh conditions (ultrasonication), the green emission of CsPbBr 3 rapidly dropped.The bright emission Table 1.The shell thickness, type of SiO 2 shell and applications of various lead halide perovskite@SiO 2 .reference shell thickness type of SiO 2 shell application [41] thick TMOS no device (photonic materials) [43] thick TMOS bioimaging and drug delivery [44] thick TMOS fluorescent sensor [45] composite TMOS down conversion white LED device (photonic materials) [46] thick TMOS down conversion white LED device (photonic materials) [47] thick TEOS fluorescent label [48] -APTES + TEOS no device [49] thick TEOS laser [50] thick TEOS laser [51] thick silica spheres + TEOS no device [52] thick silica spheres + TMOS white LED device (photonic materials) [53] silica spheres + TEOS white LED device (photonic materials) [54] thick TEOS + HMDS white LED device (photonic materials) [55] -TMOS + BN perovskite-converted LED [56] -APTES down conversion white LED device (photonic materials) [57] -APTES remote fim device (photonic materials) [58] ∼ 5 nm APTES white mini-LED device (photonic materials) [59] -APTES no device (photonic materials) [42] ultra-thin (< 1.5 nm) APTES PeLED (opto-electronic materials) [60] ultra-thin (< 2 nm) APTES PeLED (opto-electronic materials) [61] ∼ 1.5 nm APTES no device [62] (< 1.5 nm) APTES bio-imaging (photonic materials) [63] > 2 nm APTES down conversion white LED device (photonic materials) [64] -APTES no device (photonic materials) [65] thick APTES photoluminescent sensor royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 completely disappeared after 40 min.However, the bright emission of CsPbBr 3 @SiO 2 was still clearly observed after 40 min under ultrasonication.This result confirmed the improved stability compared with that of CsPbBr 3 due to the successful protection of the SiO 2 shell (figure 1b).In this study, a thick shell was obtained, which can affect carrier injection behaviour due to its insulating property.This study offers a simple and effective way to obtain highly stable CsPbBr 3 @SiO 2 core-shell nanostructures in the inorganic-perovskites-related research area.While the Zhong group [41] focused on studying the effect of OAm and OA in controlling the shell thickness during the reaction, the Park group [43] applied this method to synthesize CsPbBr 3 @SiO 2 core-shell with high luminescence and excellent water stability.These properties make CsPbBr 3 @SiO 2 core-shell compatible for bioimaging and drug delivery applications, requiring significant water and environmental stability.A cytotoxicity test showed that these core-shell perovskite nanocrystals (PNCs) were bio-compatible with nontoxicity, making them suitable for cell imaging.Furthermore, CsPbBr 3 @SiO 2 core-shell PNCs were also studied as fluorescent nanoprobes for bioimaging and drug-delivery applications.The result of this work suggests a suitable synthesis method for CsPbBr 3 @SiO 2 core-shell PNCs with wellcontrolled shell thickness for biomedical-related applications.In addition, a fluorescent sensor array based on these CsPbBr 3 @SiO 2 NCs was developed to detect sulfur-containing compounds (SCCs) such as benzothiophene (BT), dibenzothiophene (DBT), 2-methylbenzo[b]thiophene (2-MeBT), 3-methyl thiophene (3-MTP) and thiophene (TP) [44].The formation of interactions between thiophene groups and -NH 2 groups of CsPbBr 3 @SiO 2 NCs resulted in a reduction in the fluorescence signal.This type of CsPbBr 3 @SiO 2 NCs is a promising material for perovskite nanomaterial-based fluorescent sensors.In contrast to the previous studies, water was used during the synthesis process to facilitate the hydrolysis step [45][46][47].SiO 2 -coated CsPbX 3 NCs were successfully reported [45].A small amount of water was added to the hexane solution containing CsPbX 3 NCs and kept at room temperature (RT) Adapted from [41] Copyright 2018 American Chemical Society.
royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 for 8 h.TMOS as the silica precursor was introduced into the hexane solution with CsPbX 3 NCs.The presence of an amount of residual water in the hexane hydrolysed TMOS, The hydrolysis process of -OCH 3 groups on TMOS led to the successful formation of an Si-O-Si shell on CsPbX 3 NCs.It is found that the CsPbX 3 NCs will decompose by the synergistic effect of oxygen and moisture if the hydrolysis time of TMOS is too long.The XRD pattern of CsPbBr 3 @SiO 2 still maintained a cubic structure that was similar to that of CsPbBr 3 NCs without a SiO 2 shell, even after water treatment.In addition, the presence of CsPbBr 3 NCs with SiO 2 was confirmed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images along with energydispersive X-ray spectroscopy (EDS) mapping images.The interplanar distance of 0.58 Å was observed from HRTEM images.The UV-vis absorption and photoluminescence (PL) with a narrow peak (at 517 nm) of the CsPbBr 3 @SiO 2 are observed.The photostability of CsPbBr 3 @SiO 2 was much improved compared with that of CsPbBr 3 NCs.After 24 h under ambient light, the PL intensity of CsPbBr 3 @SiO 2 only dropped by 20%, whereas the PL intensity of CsPbBr 3 without SiO 2 protection decreased to 70%.An improvement in the stability against air and water of CsPbBr 3 @SiO 2 was observed due to the protective effect of the SiO 2 shell.The SiO 2 shell effectively passivated surface defects, which contributed to enhanced stability.As shown in figure 2, the CsPbBr 3 @SiO 2 -based film still showed high PL emission after immersion in water for 30 min.By contrast, CsPbBr 3 -based film started to degrade after only 60 s in water.The stable CsPbBr 3 @SiO 2 composite was used with K 2 SiF 6 : Mn 4+ to fabricate a blue InGaN chip-based LED device with a wide colour gamut.The introduction of SiO 2 shell-coated CsPbX 3 (X = Cl, Br, I) perovskite NCs is an effective and simple method to produce CsPbX 3 (X = Cl, Br, I) perovskite NCs-based optoelectronic device with high stability and good performance.
Similar to the case of the Xia et al. group's research [45], the sol-gel method was also applied by Yin et al. group [46] to synthesize CsPbBr 3 @SiO 2 NCs.After tetramethoxysilane (TMOS) was added into Cs 4 PbBr 6 NCs with hexane solution, the amount of DI water was quickly injected into this solution.This mixture was kept under ambient conditions for 12 h.Finally, the precipitates were removed to collect the product after the centrifugation step (figure 3).When the reaction time was increased, more Cs 4 PbBr 6 NCs were changed into CsPbBr 3 NCs.Besides, the size of SiO 2 also was proportionally increased with reaction time.The size of SiO 2 is from approximately 3.4 nm to approximately 12.6 nm according to the amount of TMOS that was added during the reaction.However, if the amount of TMOS exceeds the limit, it will lead to the formation of free silica molecules.These free silica molecules can affect their optical and electronic properties due to their insulating property.Thanks to the protection of the silica layer, the stability of CsPbBr 3 @SiO 2 NCs in the air, water and light treatment was dramatically enhanced compared with that of CsPbBr 3 .The CsPbBr 3 @SiO 2 NCs were still stable in the water treatment for 7 days.In addition, their emission still showed green.The particle size of CsPbBr 3 @SiO 2 NCs is much smaller than that of other reports, resulting in better dispersity in the solvent.The CsPbBr 3 @SiO 2 NCs thin films were successfully fabricated due to their excellent dispersion in the solvent, small-size NCs and high stability.The WLED  royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 devices fabricated from CsPbBr 3 @SiO 2 NCs combined with CdSe NCs and blue-emissive GaN LED chips obtained a 138% colour gamut of the National Television System Committee (NTSC, 1913) standard.This work shows that CsPbX 3 @SiO 2 -based nanomaterial is a promising material in practical applications such as white light devices application.It also provides a novel and simple approach to modifying the surface of perovskite nanocrystals with single particles and high stability, as well as controlling the shell thickness.Other SiO 2 -coated lead halide perovskite materials were also successfully produced by Maquieira et al. group [47].This group successfully studied a novel and effective synthesis for monodispersed SiO 2 -coated CsPb 2 Br 5 perovskite nanoparticles by controlling the chemical transformation of pre-synthesized CsPbBr 3 in the presence of TEOS, ammonia and water.The blue emission of the core-shell perovskite nanoparticles could still be clearly observed after dispersion in water for 3 days.By optimizing ammonia/ water ratios and reaction time, spherical CsPb 2 Br 5 @SiO 2 core-shell NPs exhibited monodispersed morphology with suitable particle size and excellent water resistance for biosensing and bioimagingrelated applications.Owing to these excellent properties, the core-shell nanoparticles were used as a fluorescent label to define bovine serum albumin (BSA).BSA was employed as a representative target protein.
In addition, the combination of APTES and TEOS was found to be effective in the construction of silica micelles, which offered well-protected CsPbBr 3 perovskite materials.CsPbBr 3 @SiO 2 perovskites were synthesized by a novel room-temperature synthesis method [48] (figure 4).The controlling -NH 2 group and -Si-O-Si-/-Si-OH in silica micelles is the main factor during the chemical reaction process.The final CsPbBr 3 @SiO 2 exhibited a photoluminescence quantum yield of 61.9% and long-term stability in ethanol.The PL stability of the CsPbBr 3 @SiO 2 ethanol sol was investigated.The bright green PL of CsPbBr 3 @SiO 2 ethanol sol still persevered for 34 days and without the occurrence of a red-shift phenomenon in the PL spectra.These results indicated the effectiveness of amine-micelles in high PL stability of CsPbBr 3 @SiO 2 sol.In summary, this work opens an effective strategy for the synthesis of long-term-stable inorganic halide perovskites in a highly polar solvent, which will be useful for various practical applications.

Silica spheres with tetramethyl orthosilicate/tetraethyl orthosilicate-based lead halide perovskites core-shell
Instead of using conventional TEOS or TMOS as a protective layer, Leng et al. group [49,50] encapsulated CsPbBr 3 QDs into waterless silica spheres based on TEOS and applied CsPbBr 3 @SiO 2 perovskites for laser applications.The hot injection method was used for synthesizing CsPbBr 3 colloidal QDs, followed by injection of the silica precursor TEOS for the growth of silica layers.After several minutes, SiO 2 layers successfully covered the surface of the CsPbBr 3 QDs.In this work, CsPbBr 3 QDs royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: and CsPbBr 3 @SiO 2 NCs exhibited similar PL spectra with a narrow green emission peak (at 522 nm) and narrow full width at half maximum (FWHM) of 18 nm.This indicated that the existence of the silica layer did not affect the optical properties.Enhanced stability in water of coated perovskite QDs by SiO 2 was observed.CsPbBr 3 @SiO 2 QDs in water still showed green light emission after 12 h, whereas for CsPbBr 3 QDs green light emission completely disappeared.In addition, the PL decay rate of CsPbBr 3 @SiO 2 film was slower than that of CsPbBr 3 film, which demonstrated enhanced moisture resistance due to the existence of the SiO 2 shell.To demonstrate the stability of CsPbBr 3 @SiO 2 QDs for lasing devices, the operation of amplified spontaneous emission (ASE) of CsPbBr 3 and CsPbBr 3 @SiO 2 under continuous excitation with pump intensity of 600 µJ cm −2 in the air over 12 h was compared.After 12 h of continuous excitation, the result showed that CsPbBr 3 @SiO 2 NCs maintained approximately 95% of light emission intensity, whereas the light emission intensity of CsPbBr 3 QDs drastically decreased to 85%.This performance indicated that the silica spheres-coated perovskite QDs performed longer operating lifetimes under the atmosphere.Furthermore, CsPbBr 3 QDs' optical properties exhibited stable ASE.Recently, the composites based on CsPbBr 3 or CsPbCl 3 perovskites and silica microspheres (MSs) synthesized using TEOS or TMOS as precursors are introduced to produce highly luminescent and stable mSiO 2 -wrapped perovskites (CsPbX 3 /MSs) for optoelectronic applications [51][52][53].Owing to protection from well-ordered silica microspheres, these CsPbX 3 /MSs perovskites displayed waterresistant and thermal ultra-stability.Besides, these perovskites' optical properties still remain in the environment for a few weeks.These mSiO 2 -wrapped perovskites showed potential as materials in the white light-emitting diode (WLED) device.The performance of WLED devices achieved significant enhancement due to the good stability and optical properties of mSiO 2 -wrapped perovskites.
Introducing a silica layer to the perovskite QDs is an effective strategy to weaken the degradation potential of perovskite nanomaterials.These studies provide a new direction for the development of perovskites/SiO 2 with excellent stability against the environment, which are attractive for stable and high-performance photovoltaic and light-emitting device applications.

Tetramethyl orthosilicate/tetraethyl orthosilicate-based lead halide perovskites core-shell with surface modification
As discussed in previous work, SiO 2 -coated CsPbBr 3 perovskites effectively protect materials against external factors.However, TMOS and TEOS-based silica shells do not contain any functional groups that can be easily modified to be compatible with other materials or device fabrication.Therefore, surface modification of CsPbBr 3 @SiO 2 perovskites is required to enhance their compatibility with various applications [54,55].Xie et al. group [54] modified the surface of CsPbBr 3 @SiO 2 perovskites as required to enhance their compatibility with various applications [54,55].Xie et al. group [54] modified them by hydrophobic groups such as hexamethyldisilazane (HMDS) to produce superhydrophobic CsPbBr 3 @SiO 2 nanoparticles and film (figure 5).The superhydrophobic SiO 2 -coated royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 CsPbBr 3 (SH-CsPbBr 3 @SiO 2 ) was synthesized by combination hydrolysis of TEOS in the presence of NH 4 OH with surface modification by HMDS.Owing to the superhydrophobicity of the hydrophobic surface, SH-CsPbBr 3 @SiO 2 films exhibited exceptional stability against water, heat and self-cleaning property.The hydrophobic surface can prevent various contaminants (dust, dirt, etc.) and help SH-CsPbBr 3 @SiO 2 films clean and retain their optical properties.These properties make this type of perovskite highly suitable for optoelectronic applications, especially those used in outdoor environments.Meanwhile, thermally conductive surface-encapsulated CsPbBr 3 PNCs were reported by Xu et al. group [55].These CsPbBr 3 PNCs prevented thermal-induced degradation caused by the low thermal conductivity of the SiO 2 layer.Importantly, the thermal degradation will hinder their practical application.Therefore, PNCs-SiO 2 -boron nitride (PNCs-SiO 2 -BN) was newly synthesized by incorporating CsPbBr 3 perovskite nanocrystals into assembled BN nanoplatelets through SiO 2 crosslinking using TMOS.Owing to the high thermal conductivity, BN nanoplatelets minimize the heat accumulation on perovskite nanocrystals in light-emitting diodes.PNCs-SiO 2 -BN exhibited good thermal stability due to the assembly structure of PNCs-SiO 2 -BN which resulted in the effective protection of PNCs at high temperatures.PNCs-SiO 2 -BN-based PcLED displayed excellent stability at approximately 0.15 W cm −2 during 1000 h of sustained illumination.

3-aminopropyl-triethoxysilane-based lead halide perovskites core-shell
The silica-based coating on APTES has been increasingly chosen as an effective protective layer for lead halide perovskites NCs.The use of APTES as a capping agent offers many advantages in the synthesis and stabilization of perovskite NCs.APTES facilitates the dissolution of PbX 2 and enhances the stability of the inorganic perovskites.
Yu et al. [56], Lin et al. [57] and Xie et al. group [58] applied a modified hot injection method to the synthesis of perovskite QDs.After adding PbBr 2 , oleic amine, oleic acid and APTES into the 1-octadecene (ODE) solution, the caesium oleate solution was injected into this solution.Then, the flask was exposed to the air at a temperature of 20°C and stirred for 3 h.During this step, APTES will undergo slow hydrolysis, royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 The -NH 2 group from APTES efficiently passivates the surface of perovskite NCs, preserving their high PLQY.The hydrolysis of the silyl ether groups results in the formation of a cross-linked matrix, encapsulating the perovskite NCs to achieve highly stable core-shell perovskite NCs against external factors.The APTES was used as a capping agent for perovskite QDs and a precursor for a silica matrix.The reaction process for the formation of perovskite QDs was carried out without the presence of even small amounts of water, which will prevent the decomposition of perovskite QDs.The successful formation of a shell was confirmed by Fourier-transform infrared spectroscopy (FT-IR).The existence of peak vibration of -Si-O-Siand a weak peak of Si-OH is related to the hydrolysis of APTES, confirming the successful formation of a SiO 2 cross-linked network.Owing to the SiO 2 shell layer, green QD/silica almost maintained the same value of origin PLQY after a long period in the atmosphere at room temperature.In Yu [56] and Xie's work [58], the QD/silica was used as a photonic material, which was combined with a blue LED chip to fabricate a WLED.The CsPbBr 0.96 I 2.04 @SiO 2 nanocrystals exhibited excellent optical properties and good stability against moisture, heat and light due to the protection of SiO 2 shell.This material, with its excellent properties such as high stability, high PLQY and anion-exchange reactions, demonstrates great potential for white lighting applications.Inspired by the work of Yu et al. [56], the modified hot injection method was also used to produce silica-coated perovskites nanocrystals (SP-PNCs) with APTES, grafted onto the surfaces of PNCs [59].The SP-PNCs exhibited high PLQY (80%) and were well-dispersed in various non-polar solvents.The well-dispersed SP-PNCs could be precipitated from their original solvents.In addition, they still retained their crystal structure, surface properties and photoluminescence quantum yield.This work provides a new greener method to synthesize SiO 2coated perovskite nanocrystals by precipitating step with polar solvents.However, in these studies, multiple particles with various sizes were encapsulated into the same SiO 2 shell.This hindered these materials in some applications, such as bio-related fields, and LED films which require a single particle.
To overcome the issue of multiple particles and achieve an ultrathin SiO 2 shell, Lee et al. group [42] synthesized green CsPbBr 3 @SiO 2 core-shell QDs with APTES as SiO 2 shell precursor by modified hot injection method with a one-step reaction.Firstly, PbBr 2 , oleic amine, oleic acid, and APTES were added into the ODE solution in an inert atmosphere.Secondly, the caesium oleate solution was injected into this solution at a temperature over 100°C.Then, the flask was exposed to the air for the hydrolysis step of the silyl ether groups in the APTES molecules to form SiO 2 shell on QDs.This hot injection method is popularly applied to produce QDs, because it is one of the most effective synthesis methods to produce uniform QDs with narrow size distribution.Importantly, this group successfully investigated various APTES/OAm ratios during the reaction to achieve an ultrathin shell, where each particle was covered by a single shell.This allowed it to be used as an emissive layer in LEDs application.TEM data (figure 6a) showed that the obtained CsPbBr 3 @SiO 2 QDs exhibited uniform size and a controlled insulating shell thickness (less than 1.5 nm).This shell thickness was sufficiently thin to exhibit effective carrier injection behaviour.The surface of QDs was effectively passivated by the -NH 2 group on APTES, which maintained high photoluminescence quantum yield royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 with PLQY approximately 70%.As the APTES/OAm ratio increased, the PLQY of CsPbBr 3 @SiO 2 QDs gradually increased, whereas the broadness of the PL peak decreased.The SiO 2 shell effectively prevented polar solvents' penetration into perovskite quantum dots, resulting in excellent chemical and environmental stability.After 10 min of adding IPA/ethanol/water into perovskites QDs, the PL intensity of CsPbBr 3 QDs without SiO 2 shell rapidly reduced due to fast desorption of the oleylammonium bromide.By contrast, the water resistance and ethanol resistance of CsPbBr 3 @SiO 2 core-shell QDs were much stronger those that of pure CsPbBr 3 .This work represents the first report of successful all-solution-processed perovskite QD-light-emitting diode (PeLED) fabrication using perovskite-metal oxide core-shell QDs as an emissive layer.Perovskite-metal oxide core-shell QDs had not previously been applied to PeLEDs due to the high carrier injection barrier of the metal oxide shell, even though it guarantees high chemical and environmental stability.However, this work successfully achieved PeLEDs using perovskite-metal oxide core-shell QDs (figure 6b) with a luminance of approximately 3200 cd m −2 owing to the ultrathin shell.The CsPbBr 3 @SiO 2 -based holeonly devices (HODs) demonstrated good charge injection efficiency with a hole mobility of approximately 1.39 × 10 −3 cm 2 V −1 s −1 .This approach is not limited to green perovskite QDs and can be extended to any type of perovskite QDs.Furthermore, the core-shell perovskite QDs also show high potential for use in various solution-processed optoelectronic applications.The introduction of these perovskite/SiO 2 core-shell QDs is a simple and effective method to produce new perovskite core-shell with the ultrathin shell, which is useful for perovskite QD-based optoelectronic devices.
In addition, Lee et al.
[60] also reported red CsPbI 3 @SiO 2 core-shell QDs with APTES as the shell via a modified hot injection method.Owing to successful control over the shell thickness and gradient I doping, CsPbI 3 @SiO 2 core-shell QDs could be applied to optoelectronic devices.Based on the result of TEM data, the SiO 2 shell thicknesses of QD/0.8APTES and QD/1.0APTES were approximately 1 nm and approximately 2 nm, respectively, which are thin enough to exhibit effective carrier injection behaviour (figure 7).The mechanism of degradation of red perovskite under electric fields was investigated to solve the problem of the low operational stabilities of PeLEDs.The operational stability of the red perovskite core-shell QDs-based PeLEDs (QD/0.8APTES)was approximately 5000 times compared with those of red pristine-based PeLEDs.This indicated their high stability against temperatures and electric fields due to the protection of the SiO 2 shell, which resulted in effectively passivating the surface defects.These studies provide a useful strategy for designing new red perovskite/SiO 2 core-shell QDs with the ultrathin shell for optoelectronic devices.They demonstrated the potential to have a breakthrough in perovskite QD-based electronics such as LEDs, solar cells, thin film transistors and other optoelectronic applications.This study is not only applied to red perovskites, it can also be extended to other types of perovskites with different anions.In contrast to the synthesis method used by Lee et al. [42,60], Yoon et al. [61] employed a modified ligand-assisted reprecipitation technique to encapsulate CsPbBr 3 perovskites QDs with an ultrathin silica shell (APTES) that would facilitate effective charge transport (figure 8).The N,N-dimethylformamide solution containing the perovskite precursors was poured into an antisolvent (toluene) to induce the royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 precipitation of perovskite quantum dots.Afterwards, their precipitate was put in ODE at 225°C.This resulted in the formation of a silica protective layer against water.Owing to the narrow shell thickness of approximately 1.5 nm, this SiO 2 shell could be useful for the electron/energy process from the core to the electron transport media through the Dexter energy transfer or electron transfer.Therefore, the transfer process should be used for the application of CsPbBr 3 @SiO x perovskite QDs in various areas such as optoelectronic devices and solar-driven chemistry.This study provides valuable insights into the core-shell perovskites covered with silica shells in renewable energy applications.
To enable the utilization of perovskite materials in bio-related applications, Song et al.
[62] synthesized CsPbBr 3 @SiO 2 nanoparticles by using water during the reaction, instead of using the hot injection method without the presence of water.The SiO 2 shell thickness was easily controlled by the hydrolysis process time.The synthesis process of CsPbBr 3 @SiO 2 nanoparticles has two steps.The water was added into a solution containing CsPbBr 3 /APTES/toluene, then precipitation of this solution with methyl acetate solvent obtained CsPbBr 3 @SiO 2 nanoparticles.The shell thickness increased from 1.5 to 3.5 to 4.2 nm as hydrolysis time increased from 5 to 10 to 30 min.However, if hydrolysis exceeded 1 h, serious aggregation was observed due to excessive APTES hydrolysing to form bulk SiO 2 .Thanks to the shell formed on the surface of CsPbBr 3 , CsPbBr 3 @SiO 2 still showed bright emission in water for 48 h.By contrast, CsPbBr 3 without a SiO 2 shell was highly unstable in a water environment.Its PL features completely disappeared in water after a duration of less than 1 h.The CsPbBr 3 @SiO 2 nanoparticles exhibited great photoluminescence, high water stability, biocompatibility characteristics and low cytotoxicity.These CsPbBr 3 @SiO 2 nanoparticles were used as the fluorescent probe for CT26 tumour cell imaging in this work.This research showed the potential of lead halide perovskites core-shell for tumour diagnosis, for the new generation of optical probes in various biomedical applications such as cancer cells.
Different from the synthesis methods mentioned above, Zang et al. [63] reported one-step roomtemperature synthesis to coat CsPbBr 3 QDs with APTES.This entire process only took a short time (20 s) at room temperature.This method was simple and did not require additional water or prolonged stirring.Therefore, the QDs will be prevented from decomposing by water before SiO 2 formation.The solution containing PbBr 2 , CsBr, OA and OAm was quickly added to toluene containing APTES at room temperature.During the hydrolysis of APTES, an SiO 2 shell was formed via the interaction between QDs surfaces and APTES.XRD patterns showed that the CsPbBr 3 @SiO 2 QDs exhibited an orthorhombic structure, similar to that of CsPbBr 3 QDs.This proved that the SiO 2 shell has no influence on the crystallinity of CsPbBr 3 QDs.In addition, the CsPbBr 3 @SiO 2 QDs also exhibited excellent stability against oxygen, moisture, heat and polar solvent (ethanol).The stability of CsPbBr 3 QDs and CsPbBr 3 @SiO 2 QDs films was tested under heating temperature at 80°C.The PL intensity of the CsPbBr 3 @SiO 2 QDs slowly decreased and still remained at 53% value of PL intensity after 60 min of heating at 80°C.By contrast, CsPbBr 3 QDs rapidly declined after a duration of 60 min.Similar to the stability under heating, after 30 min with ethanol, CsPbBr 3 @SiO 2 QDs still maintained 87% of PL intensity, whereas the bright emission of CsPbBr 3 QDs disappeared.This confirmed that SiO 2 -coated CsPbBr royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 of the SiO 2 shell on the surface of QDs.The CsPbBr 3 @SiO 2 QD was used as a photonic material with red Ag-In-Zn-S QDs on InGaN blue chips to fabricate WLEDs.These WLEDs exhibited excellent luminescent performance with a power efficiency of 40.6 Lm W −1 .This work provided SiO 2 -coated CsPbBr 3 core-shell perovskite QDs as potential photonic materials with high PLQY, and good chemical, as well as stability, which are suitable for highly efficient (high colour-rendering thermal index) and stable WLEDs devices.This demonstrates their potential applications in display and solidstate lighting fields.
Furthermore, Wei et al. group [64] reported a modified ligand prepared from bis[3-(triethoxysilyl) propyl] amine with gultaric anhydride (BTPA-GA) and APTES to passivate the CsPbBr 3 @SiO 2 QDs surface.After the QDs were dispersed in a polystyrene (PS) matrix and under heat treatment, Si-O-Si cross-linking of the surface ligands was formed to protect CsPbBr 3 .The PS matrix was used to interweave the CPB@SiO 2 and prevent aggregation during the formation of CPB@SiO 2 .The CPB@SiO 2 /PS film was dissolved in toluene and washed three times to remove the PS matrix.A cross-linked Si-O-Si organic silica network formed from the hydrolysis process of ethoxyl silane ether groups and condensation reaction under heating.Meanwhile, the -NH 2 group of APTES also controlled the size of QDs.In addition, the -COOH group of BTPA-GA suppressed the aggregation of QDs and affected the coordination between the -COOH group and Pb.The size of CsPbBr 3 QDs without PS matrix was larger and exhibited a more irregular morphology compared with that of the CPB QDs.Introducing a SiO 2 protective layer onto the surface of CPB@SiO 2 enhances its stability when exposed to acid-base environments and polar solvents.The PL peak intensity of CPB@SiO 2 did not change much in DMF over the duration of one month.In addition, CPB@SiO 2 still survived in ammonium hydroxide or tetrabutylammonium hydroxide, or acid (acetic).Owing to excellent properties of stability in polar solvents, water and acid-alkaline conditions, this material was used as solution-processable luminescent inks.A five-letter 'ECUST' pattern written using CsPbBr 3 @SiO 2 solution in toluene as ink exhibited excellent uniform, strong PL emission and survived until 100 s.By contrast, the 'ECUST' letter pattern was prepared from SiO 2 -uncoated CsPbBr 3 solution as the ink faded.This work suggested a new method to produce core-shell perovskites with excellent stability in various conditions and became promising materials in the ink field.In addition to bio and optoelectronics applications, perovskite@SiO 2 with excellent luminescence performance, stability and dispersion in ethanol was applied in sensing applications.Particularly, in Chen et al. group [65], CsPbBr 3 @SiO 2 PNCs were used in the fluorescence sensing of Cl − in sea sand samples.This group successfully developed CsPbBr 3 @SiO 2 PNCs by an efficient synthesis method with the support of a nucleophilic substitution strategy using benzylic bromide.This process opens up new possibilities for the expansion of perovskite@SiO 2 nanocrystals-based fluorescent sensors.

Conclusion
The lead halide perovskite materials-based applications have drawn the attention of scientists for stable inorganic halide perovskites and high performance of the device.The lead halide perovskites have excellent optical properties, which will be useful for optoelectrical and bio-related fields.However, their stability against external factors such as environment or polar solvents is very poor.This limitation has hindered the practical applications of these materials.Therefore, various materials containing SiO 2 groups, such as TMOS, TEOS and APTES, were introduced to stabilize the perovskite.The encapsulation of SiO 2 as the shell helps dramatically improve the endurance of perovskites under environmental and/or chemical conditions.This also helps increase their high PLQY with narrow sizes.In addition, the shell thickness can be well controlled so that these materials can be applied for various applications such as PeLED, WLED, bio-imaging and drug delivery.Enhancement of environmental and chemical stability of inorganic perovskites as well as controlling the thickness shell of inorganic halide perovskites core-shell play an important role in developing new types of halide perovskites for practical applications.This review will be useful in guiding researchers to find the suitable method for fabricating stable and highly efficient perovskite core-shell-based devices.
However, there are still some remaining limitations of SiO 2 -coated lead halide perovskite core-shells: 1. Conditions applied in the process of the growth of the shell by hot injection method often need an inert environment, high temperature, precise timing and sophisticated equipment.Even though this method can produce uniform perovskite core-shell, this requirement is a barrier to commercialization due to the less effectiveness during mass-scale production.
royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 230892 2. Heavy metals from the decomposition of the perovskites@SiO 2 after long-term storage in the air will affect the environment.This will limit their applications (bio-related fields, etc.).3. Perovskite core-shell shows great potential in many applications (optoelectronic, bio-imaging, etc.).
But, the separation and extraction of excited charge carriers can be inhibited due to their own structure.This will hinder the material in some applications such as solar cells.Variety of new perovskites core-shell should be studied.
Thus, we suggest a synthetic process that involves lead halide perovskite core-shell with greener solvents, instead of common organic solvents during the formation of the SiO 2 protective layer and device fabrication.Furthermore, we can also apply this type of silica-based core-shell structure to halide lead-free perovskites, which will not be harmful to human health and the environment.This will open new chances for the development of eco-friendly perovskite materials in bio-related, optoelectronic and other related fields.

Figure 4 .
Figure 4.The illustration of the room-temperature synthesis process of CsPbBr 3 @SiO 2 within APTES and TEOS in ethanol.Adapted from[48] Copyright 2020 American Chemical Society.